1. Field of the Invention
The present invention relates to a spindle unit of an apparatus with a rotating unit such as, for example, a magnetic disk apparatus, an optical disk apparatus and a laser beam printer and, more particularly, to a spindle unit adapted to stably be driven for rotation and preventing contamination of a disc.
2. Description of the Related Art
With the down sizing or miniaturization of computers, the down sizing of a magnetic disk apparatus as an external memory device is also required. On the other hand, demands for a large capacity and a high processing speed also continuously exist. To satisfy those demands, a magnetic disk apparatus of a small size and a large capacity has been realized by increasing a recording density. For this purpose, it is necessary to accomplish a high performance and a small size of a spindle unit rotating at a constant speed.
In the conventional spindle unit, a ball bearing is used for a supporting mechanism of the spindle. By increasing working precisions of the ball bearings and inner and outer wheels, an irregular rotational fluctuation due to the ball passage vibration that is peculiar to the ball bearing is reduced, resulting in an improved rotating precision. However, since an adequate precision cannot be achieved only through the improvement of the working precisions as mentioned above, the use of the sliding bearing is considered. In the sliding bearing, it is necessary to mechanically apply pre-loads in the radial and thrust directions to position the spindle and thereby to improve the rotating precision.
In for example, JP-A-61-151877, a technique is proposed wherein, in order to apply the pre-loads, ball bearings are provided in the upper and lower portions of the spindle, with the outer wheel of the upper bearing being fixed to the bearing housing at the upper portion and the outer wheel of the bearing at the lower portion being preloaded in the thrust direction by a spring member. The and outer wheel of the lower ball bearing is guided in a concave portion of the bearing housing by an attracting force in the radial direction of the spindle by a pad-like permanent magnet provided between the two ball bearings in close vicinity to the spindle, resulting in stable rotation of the spindle. However, a micro vibration due to a rolling contact between the outer wheel surface and the balls or the inner wheel surface and the balls of the ball bearing cannot be prevented. In addition, since a motor section for driving the spindle is provided on the lower side of a magnetic disk enclosing section so as to be projected therefrom, there is a problem that the apparatus is long in the axial direction and increases in size.
JP-A-60-5467 discloses a technique to suppress the vibration by a magnetic attracting force between a concentrical magnetic material provided in a part of the main shaft system and a permanent magnet provided on the fixed side so as to face the magnetic material. According to such a technique, however, there is a problem that, although the vibration in the axial direction can be suppressed, a vibration suppressing effect in the radial direction is insufficient. There is also a problem that, the shape is large because a magnetic force operating section, a pulley (rotation driving section), and the like are projected to the lower side.
JP-A-2-180312 proposes a technique wherein, in order to reduce the vibration by setting the center of gravity of a rotating body to a low position, the rotating body which is heavier than the rotor of the motor is attached to a position lower than the position of the rotor, and further the rotating body is lifted up in the axial direction by a magnetic force between a ferromagnetic material provided for the rotating body and a magnet attached to the housing side, thereby reducing a load of shaft. There is, however, a problem that the rotating body is projected to the lower portion of the spindle unit thereby increasing the overall size and increasing the weight of the rotating body.
JP-U-63-130958 proposes a technique wherein an eccentric member in the form of a permanent magnet provided between the bearings fixed to a bearing housing, is fitted into a supporting hole formed in a side wall portion of the bearing housing locating on an extension line in the seeking direction of a head, and an adjustment screw is further provided for a supporting hole, thereby adjusting the position between the spindle and the eccentric member. Therefore, the spindle is attracted to the eccentric member such that it contacts and is supported by the inner surface of the bearing. The eccentric position of the spindle corresponds to the seeking direction of the head, and the spindle moves along an elliptical orbit with the minor axis in the eccentric direction. Therefore, a non-rotating period oscillation decreases and a tracking error upon reproduction is decreased. However, a space to adjust the position of the eccentric member is necessary. Since the eccentric member is provided on the side wall portion of the bearing housing with a sealed lubricating agent, there exists the possibility of lubricant leakage from its opening portion.
JP-A-4-12 proposes a technique wherein a magnet, magnetized in the vertical direction over a range of 180.degree., is arranged between the bearings for rotatably supporting the shaft member formed of a magnetic material and the shaft member is brought into line contact with one side of the bearing member by an attracting force of the magnet, thereby stabilizing the rotating motion. However, since no magnetic circuit is formed on the side having no magnet, the lubricating oil cannot be prevented from scattering to the outside from the sliding surface of the bearing due to a centrifugal force during the rotation of the shaft. There is, consequently, a problem that a lack of oil occurs on the sliding surface and the bearing is burnt and damaged in a short period of time.
Generally, in a rotating apparatus such as magnetic disk apparatus, optical disk apparatus, or magnetooptic disk apparatus which is extremely sensitive to a contamination of the disk, a sealing device to prevent the scattering of the lubricating oil for the bearing is provided at an edge surface of the bearing. In many instances, a magnetic fluid seal is used in the sealing method. When the spindle is rotated at a high speed, the magnetic fluid (lubricating oil) is often scattered due to the centrifugal force in association with an increase in rotational speed. The reliability of the magnetic fluid sealing device itself causes a problem. As disclosed in JP-A-60-175870 and JP-A-60-245878, conventional magnetic fluid sealing devices, pole pieces of multistages are provided for a shaft, a permanent magnet and a magnetic pole member are arranged on the stationary side, and a magnetic circuit is constructed by a ball bearing and a magnetic pole member, thereby providing a compact sealing device. As disclosed in JP-A-60-249727, JP-A-64-79420, JP-U-64-725, and JP-U-1-87330, there is also a sealing device which is constructed by arranging a magnet and a pole piece to the outer wheel or inner wheel of a ball bearing. Further, as disclosed in U.S. Pat. No. 4,630,943, a magnet and a pole piece are provided on the stationary side on the inner peripheral side than the bearing diameter, thereby improving the sealing function. Further, as disclosed in JP-U-63-42928, a structure is proposed wherein a projecting portion is provided on the shaft side and a magnet and a pole piece are arranged on the stationary side, thereby making it difficult for the magnetic fluid to be scattered by the centrifugal force. On the other hand, as disclosed in JP-U-64-54748, there is also a structure such that a cover to prevent the scattering of the magnetic fluid is attached to the outer peripheral portion of the pole piece.
However, for example, according to the techniques disclosed in JP-A-60-175870 and JP-A-60-245878, where the magnetic fluid is scattered during the rotation at a high speed, the magnetic fluid is attracted to the magnet and it is difficult to collect the magnetic fluid at the pole piece. On the other hand, according to the techniques disclosed in JP-A-60-249727, JP-U-64-79420, JP-U-64-725, and JU-A-1-87330, no consideration is made with respect to the scattering of the magnetic fluid. In the technique disclosed in JP-U-64-54748, although a cover is provided for the preventing scattering of the magnetic fluid, the magnetic fluid collected to the cover is merely held on the side surface of the pole piece and is hardly collected into a seal gap portion at the tip of the pole piece. Even in case of the structure such that the magnetic fluid is hardly scattered due to the centrifugal force as shown in the technique disclosed in JP-U-63-42928, since a magnetic flux density at the position where the magnetic fluid is held is low, a seal withstanding pressure is low and no consideration is made for the scattering in the axial direction.